1. Field of the Invention
The present invention relates to a solid-state image pickup element and a solid-state image pickup device.
2. Related Art
Because of spread of the digital camera and the cellular phone with built-in camera in recent years, demands for the solid-state image pickup element are increasing. In particular, demands for the CMOS solid-state image pickup element which can be manufactured by the CMOS process which is a general semiconductor manufacturing process arise. In recent years, such a solid-state image pickup element is required to have a smaller size and a larger number of pixels. It has become an important issue to make the pixel size smaller.
As compared with a reduction of a plane size parallel to the semiconductor substrate surface consequent upon the smaller pixel size, however, a reduction in a depth direction perpendicular to the semiconductor substrate surface is not scaled at the same time. Problems caused by this are being actualized.
In other words, even if the size of a pixel area in the horizontal direction is reduced, the distance between the silicon substrate surface on which a photodiode is formed and a microlens for optically expanding the aperture ratio is not reduced. Therefore, a part of incident light is reflected by a metal wiring layer, in a pixel area peripheral part where incident light from an image forming lens is incident aslant. In other words, the so-called “eclipse” or “vignetting” phenomenon occurs in the pixel area peripheral part, and sensitivity nonuniformity occurs.
Furthermore, if it is attempted to mount a circuit designed to use the latest CMOS process on a peripheral circuit in order to implement “system on chip” which is a feature of the CMOS solid-state image pickup element, then the distance between the silicon substrate surface and the microlens is made long by multi-layer wiring in the CMOS process, and the “eclipse” or “vignetting” becomes further apt to occur.
As a device configuration for breaking down such a situation, development of a back-illuminated CMOS sensor is reported (see, for example, Japanese Patent No. 3759435 and S. Iwabuchi et al., ISSCC Tech. Digest, pp. 302-303, 2006).
In such a back-illuminated CMOS sensor, the silicon substrate must be thin in order to efficiently collect signal electrons generated by incident light from the back side absorbed at the silicon substrate. For example, in the aforementioned S. Iwabuchi et al., ISSCC Tech. Digest, pp. 302-303, 2006, the silicon substrate is formed as a thin film having a thickness of 4 μm.
A problem posed by the silicon substrate formed as the thin film becomes remarkable when the incident light intensity is high, i.e., when an image of high light is taken. In other words, if excessive electron-hole pairs are generated by incidence of the high light, then excessive signal electrons which have exceeded the saturation charge quantity of the photodiode overflow and are injected into a photodiode of an adjacent pixel, resulting in a problem of blooming or crosstalk.
In a method proposed against this problem, a channel under a transfer gate is utilized as a path for lateral overflow by providing a transfer transistor in which a photodiode and floating diffusion are separated in a storage period with a gate voltage condition which does not bring the transfer transistor into a complete off state.
When this method is used, however, the potential of the overflow channel varies in channels under the influence of the threshold variation of the transfer transistor which is a MOS transistor and consequently the potential variation appears in the output at the time of saturation as a fixed pattern.
On the other hand, a method of forming an overflow channel for excessive signal electrons from the photodiode in a bulk area is also proposed under the notion that the potential variation is caused by the MOS structure (see, for example, JP-A 2006-49338 (KOKAI)).
In this method, however, the barrier height of the overflow channel is controlled by using a silicon substrate impurity concentration which is an extremely low concentration, and consequently its control is substantially difficult.